The majority of contemporary electronic musical synthesizers use digital circuitry to generate tones in preference to analog circuitry, which was widely used until the early 1980's when digital techniques became economically feasible. Digital methods of tone generation have advantages of reduced cost, ease of manufacture and increased functionality, for example, the ability to synthesize the sounds of several different instruments simultaneously. A sequencer program may control a tone generator capable of producing the sounds of a drum kit, piano, strings and brass. Thus a musician may compose and preview complex arrangements without the delay and cost of using real musicians. Such facilities are a major reason for the widespread popularity of musical synthesizers which incorporate digital tone generators.
Unlike analogue electronic tone generators, most digital tone generators do not naturally produce sounds which are aesthetically appealing. The direct transposition of analog tone generator building-blocks, such as oscillators and filters, into the digital domain has proved unsuccessful. Thus digital tone generators largely exploit the ability to play back recordings of real musical instruments and passing such recordings or "samples" through filters and effects to enhance their appeal. The amount of memory required to store samples of a real instrument at the pitch of every note on a keyboard is excessively large, therefore a smaller number of samples are used, with a sample for a particular note speeded-up or slowed-down in order to simulate the sound of the instrument played at different pitches. An additional technique for saving sample memory is "looping", when a portion of the recorded sound is played back several times within the duration of a note. Sounds such as those of a string ensemble may particularly exploit this technique, as looping is less noticeable when employed in a sound of long duration and relatively constant amplitude.
Sampling is inherently limited by the ability of the human ear, especially the trained musical ear, to recognise the harmonic signatures of musical instruments. When a sample has been speeded-up, slowed-down or looped, the ear perceives resulting changes in pitch or distortion as unnatural, and may find the synthetic sound less appealing than the sound of a real instrument.
A further limitation of known sampling techniques is that variations in the tone produced is restricted by the amount of memory available for sound storage, such that it is impossible to store the entire range of tones produced by a real instrument, such as a saxophone. Dynamic changes in tone throughout the duration of a note can only be approximated very roughly by the application of synthetic filtering techniques.
In recognition of the restriction of existing digital musical instruments based on sampling, considerable research has been done in an attempt to mathematically model the physical processes that occur in a real musical instrument. Efficient mathematical models of physical processes can be used in algorithms for digital signal processors which can generate sounds in real time. If the physical processes of sound production are modelled in sufficient detail, it is possible to control aspects of the model to produce the same kind of changes in tone that may be produced on a real musical instrument. Furthermore, physical modelling can be used to define models of musical instruments which are physically impossible to build, and in this respect physical modelling may have considerable potential for the innovation of new sounds.
Physical modelling requires considerable digital signal processing circuitry in order to simulate the sound of a single note played on a real instrument. Thus electronic musical instruments based on physical modelling are very expensive and will remain so until digital signal processing becomes much cheaper. An additional problem with physical modelling is that defining the properties of a musical instrument in their entirety is beyond the ability of musicians who would use such an instrument. Thus analogue synthesizers, which may be controlled intuitively during a live musical performance, still retain a considerable advantage over the most recent generation of electronic synthesizers.